1. Field of the Invention
The invention relates to the field of electrochemical gas sensors having a consumable electrode, and particularly to a method for testing an operational sensor to determine if it is near to its end of useful life.
2. Description of Related Art
Over the last thirty years, instruments have become available for monitoring workplace atmospheres for hazardous gases. Atmospheres may be hazardous because of the presence of toxic gases, or combustible because of a deficiency or excess of oxygen. These gas detection instruments typically contain a gas sensor producing an electrical output signal which varies as a function of the gas concentration, and electronics to drive the sensor and to amplify and manipulate the output signal to give an auditory or visual warning or both in the event of a potentially dangerous atmosphere. Many of the present day instruments have digital displays and give a continuous output showing the gas concentrations of interest and often incorporate microprocessor controls, thus allowing more advanced features such as data logging, calculation of time weighted average exposures.
The concentration of oxygen is especially important, since if the concentration falls significantly below normal atmosphere (21% v/v at 1 atm. pressure) then insufficient oxygen will be absorbed by the blood in the lungs, resulting in decreasing oxygen concentration and impairment of judgement, nausea, vomiting, inability to move freely or cry out, and eventually convulsive movements and death (L. R. Cooper, Oxygen Deficiency in Detection and Measurement of Hazardous Gases, Ed. C. F. Cullis, J. G. Firth, Heinemann, London, 1981). If the oxygen concentration is too high, then the combustion of many flammable materials is facilitated, which also presents a possible hazard. Oxygen detecting instruments often have both an upper and lower concentration alarm level, typically at about 25 and 19% volume respectively.
The most common type of oxygen sensor used in instruments for monitoring workplace safety is an electrochemical sensor. The theory of operation and practical usage of electrochemical gas sensors has been discussed in detail by Chang et al. (S. C. Chang, J. R. Stetter, C. S. Cha, Talanta, Amperometric Gas Sensors (1993), 40, 461) and by Hobbs et al. (B. S. Hobbs, A. D. S. Tantram, R. Chan-Henry in Techniques and Mechanisms in Gas Sensing, Ed. P. T. Mosely, J. Norris, D. E. Williams, (1991).)
Amperometric electrochemical sensors contain at least two electrodes in contact with an electrolyte. Oxygen diffuses into the sensor through a diffusion barrier to one of the electrodes, known as the cathode. The electrons required for the reduction of the oxygen flow through the external circuit from the anode, where an equal magnitude oxidation reaction occurs. This flow of electrons constitutes an electric current, which provides the output signal. The potential of the cathode is selected such that all the oxygen which reaches the cathode is electrochemically reduced. This potential may be established by application of an external potential, thus operating the sensor in so-called polarographic mode, or by use of an anode material which is sufficiently electronegative in is the electrochemical series, such as lead or cadmium. A sensor of this latter type is known as a galvanic oxygen sensor, examples of which have been described, for example, in Lawson, U.S. Pat. No. 4,085,024, Tantram et al, U.S. Pat. Nos. 4,132,616 and 4,324,632, Culliname in U.S. Pat. No. 4,446,000, Bone et al, U.S. Pat. No. 4,810,352 and Fujita et al, U.S. Pat. No. 4,495,051.
A polarographic sensor requires an external circuit to control the potential of the sensor electrodes at a fixed value, whereas the galvanic sensor can be operated by simply placing a load resistor between the two electrodes and measuring the potential difference across this resistor, which is proportional to the current flowing through the resistor. Galvanic sensors may also be operated with a potentiostat circuit, which fixes the potential between the two electrodes. For most galvanic sensors operated in this mode, the applied potential will be zero, but other potentials may also be used.
Oxygen sensors are well known in the prior art, and polarographic and galvanic sensors have both been widely used for measuring the oxygen concentration in both gases, especially air, and in liquids (M. L. Hitchman, Measurement of Dissolved Oxygen, John Wiley & Sons, N.Y. 1978; I. Fatt, Polarographic Oxygen Sensors, Its theory of Operation and its Application in Biology, Medicine and Technology, Robert E. Krieger Publishing Company, Malabar, Fla. 1982).
In a typical galvanic sensor, the flow of electrons from the anode is generated by the oxidation of the anode material. For a lead anode, the reaction is believed to be oxidation of the lead to form lead oxide (PbO). The rate of oxidation depends on the amount of oxygen being reduced, which in turn depends on the rate of diffusion of oxygen into the sensor through a diffusion barrier. Since the rate of diffusion depends on the concentration of the oxygen outside the sensor, external oxygen concentration.
Since the anode is consumed in a galvanic sensor during the detection process, the sensor has a finite lifetime. Once all of the anode material has been consumed, the sensor will no longer detect oxygen. The output current of a working sensor is limited by the rate of diffusion of the oxygen into the sensor via the diffusion barrier and so the output current is independent of the state of the anode. Once the anode is consumed, then the sensor will fail and this failure often occurs rapidly, with little or no warning. When the sensor fails, the output current decreases. However, a fall in output current can be due either to a failed sensor or to the gas detection instrument being in an environment with a reduced oxygen concentration. Thus, there may be confusion about whether the sensor has failed or the oxygen concentration has decreased; this confusion is at the least very annoying and potentially dangerous.
Therefore, a method is needed to predict when a sensor will fail, before it actually does, so that a warning can be provided to the user in advance. Early warning of imminent sensor failure will allow the sensor to be replaced before it fails.
The ability to determine whether the sensor is working correctly, or to predict imminent failure is an important advantage for an instrument used for safety applications. These various problems outlined above have been addressed in the prior art to various levels of satisfaction. The most common method of ensuring that gas sensors are working correctly is frequent and periodic calibration.
Calibration is usually performed manually, by the application of calibration gases of known composition, or by exposure of the gas detection instrument to clean air. Automatic calibration methods have been described in the prior art, for example, Stetter et al in U.S. Pat. No. 4,384,925, Hyer and Roberts in U.S. Pat. No. 4,151,738, Hartwig and Habibi in U.S. Pat. No. 5,239,492 and Melgaard in U.S. Pat. No. 4,116,612 describe methods for automatic calibration of a gas detection instrument in which calibration gas are automatically applied to the sensors under the microprocessor control.
Calibration methods have also been devised in which the test gas is generated as needed, such as the electrochemical gas generators used by Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical Information Sheet, titled A world of gases . . . A single, transmitter) to provide test gas to automatically check the performance of gas detection instruments, and ensure that the sensors are responding within their specified limits. Finbow et al. in U.S. Pat. No. 5,668,302 discloses incorporating an electrochemical gas generator within an electrochemical gas sensor, behind the diffusion barrier, to provide a means for automatic function testing of the gas detection instrument.
Other methods have been devised which can achieve calibration without prior knowledge of the gas concentration, based on application of Faraday's law of electrolysis to a known volume of gas, described by Tantram and Gilbey in U.S. Pat. No. 4,829,809 and by Matthiesen in U.S. Pat. No. 4,833,909; these methods do not require a known test gas concentration.
Calibration is a very important process in gas detection, but does not provide any warning of imminent failure of a galvanic oxygen sensor. Since the failure can occur rapidly, the sensor can be successfully calibrated, only to fail a short time thereafter. Clearly a better method of determining the status of the galvanic sensor is required.
Other approaches have focused on the electrical properties of the sensor; for example Jones, U.S. Pat. No. 5,202,637 and Studer, U.S. Pat. No. 5,611,909 apply a small potential perturbation to the normally constant potential between the reference electrode and the working electrode and monitor the electrical current response of three electrode toxic gas sensors. Doer and Linowski have also described related electrical tests for HPLC electrochemical detectors in U.S. Pat. No. 5,100,530. While providing a simple, in-situ test that an instrument or controller can automatically perform on the sensor, this method will only detect those modes of sensor failure which affect the electrical properties of the working electrode, such as loss of volume due to dry-out from an aqueous based electrolyte.
Unfortunately, the failure modes of three electrode toxic gas sensors, which typically do not use consumable electrodes, are quite different from galvanic oxygen sensors, and therefore the tests described are not applicable to the latter and furthermore, the majority of these tests only indicate whether a sensor is working or not at the time of the test. What is needed is a predictive test that will provide a warning that a galvanic sensor is about to fail, but to give this warning while the sensor is still working.
A diagnostic test for exhaust gas oxygen pump sensors was described by Wang et al in U.S. Pat. No. 5,558,752. The apparatus used includes a pair of two electrode, solid, non-consumable electrolyte cells, one cell being used as an oxygen pump and the other cell providing an output signal. The test involves applying a pertubation to the potential across the pump cell, such as a low amplitude alternating current, and the resulting alternating current component of the pump cell current is used to determine if the output from the sensing cell is outside of the current limiting range of operation. This method will detect whether the sensor is no longer functioning correctly, but will not predict if the sensor will fail in the near future.
One method which has been described by Tantram et al. in U.S. Pat. No. 4,132,616 involves modifying the sensor design to include a small amount of a second metal, such as copper, within the anode, which is more electropositive than the lead, but which is still sufficiently electronegative to provide the potential to the cathode for the reduction of oxygen. Once all of the lead has been consumed, the open circuit potential of the sensor will differ between lead as the active anode material and copper as the active anode material. This method has the advantage that it is a predictive test, such that the user is warned prior to the failure of the sensor. However, this test requires modification of the galvanic oxygen sensor design, and requires sufficient time for the sensor to reach steady state open circuit potential, which may be a considerable length of time. A test which could be applied to any galvanic oxygen sensor would be much more advantageous, especially if this test could be performed periodically by a gas detection instrument with little or no need for human intervention.
Another method for predicting the failure of a galvanic oxygen cell was described by Parker in U.S. Pat. No. 5,405,512, in which the sensor is modified to include two or more anodes with a switching circuit to connect them in turn to the single cathode. While this method does provide a predictive test for failure of the galvanic oxygen sensor, it also requires the use of a novel and complex sensor design, and is not compatible with most of the sensors currently in use.